The most severe consequence of the aging brain is dementia, which is defined in the Diagnostic and Statistical Manual of Mental Disorders, 4th edition (DSM-IV) as                “the development of multiple cognitive deficits that included memory impairment and at least one of the following cognitive disturbances: aphasia, apraxia, agnosia, or a disturbance in executive functions. The cognitive impairment must be sufficiently severe to cause impairment in occupational or social functioning and must represent a decline from a previously higher level of functioning.” [1]        
The number of elderly people is increasing rapidly within our society and as a consequence, dementia is growing into a major health problem. In 1991, the Canadian Study of Health and Aging had estimated 25% of the population over the age of 65 had a form of dementia. The study also estimated the number of people living with dementia will double and triple in Canada by 2011 and 2031, respectively [2].
The clinical manifestation of dementia can result from neurodegeneration (e.g. Alzheimer's Disease [AD], dementia with Lewy bodies [DLB] and frontotemporal lobe dementia [FTLD]), vascular (e.g. multi-infarct dementia) or anoxic event (e.g. cardiac arrest), trauma to the brain (e.g. dementia pugilistica [boxer's dementia]), or exposure to an infectious (e.g. Creutzfeldt-Jakob Disease) or toxic agent (e.g. alcohol-induced dementia) [3].
AD is the most common cause of dementia, followed by vascular dementia (VaD), DLB and FTLD [4]. The differential diagnosis of the types of dementia is not straightforward, and is typically based on exclusion of other disorders [5]. For example, blood chemistry values are measured to determine if Vitamin B12 deficiency, anemia, infection, venereal disease or thyroid disorder may be possible reasons for the dementia symptoms. Various neuroimaging techniques may be employed, such as magnetic resonance imaging or computerized tomography scans to determine if the symptoms may be due to the presence of a tumor, infection or vascular event [4].
If the dementia symptoms can not be explained by another disorder, a diagnosis of AD, DLB or FTLD is made exclusively based on the clinical symptoms (e.g. frequency of falls, rapid onset, presence of visual or auditory hallucinations, etc). It is not until a histopathological evaluation of the brain during autopsy is performed that a definitive diagnosis can be obtained [5-7]. A prospective study on the prevalence of AD in people over the age of 85 indicated that more than half of the individuals with neuropathological criteria for AD were either non-demented or were incorrectly diagnosed with VaD. As well, 35% of those individuals diagnosed with AD based on clinical features were incorrectly diagnosed as the neuropathological evaluation did not support that diagnosis [8]. The degree of misdiagnosis is understandable since the clinical symptoms of the various dementias often overlap and is dependent upon whether the pertinent information is made known to the clinician.
The different types of dementias are based on specific neuropathological features. A definitive diagnosis of AD relies on the deposition of two types of neuronal protein: tau in the form of intraneuronal neurofibrillary tangles (NFTs) and the accumulation of extracellular β-amyloid to form senile plaques (SPs). Tau is important for the formation of microtubules in the neuronal axon by binding and promoting polymerization of tubules. In AD, tau becomes hyperphosphorylated thereby disrupting its main function. The tau accumulates and forms tangles within the axon. The neuron can no longer function and dies. Tau protein is released into the extracellular space where it can be detected in the cerebrospinal fluid (CSF) [9]. The formation of SPs, however, is due to the accumulation of a 40 and 42 residue protein β-amyloid from amyloid precursor protein (APP) [10]. The formation and secretion of β-amyloid is closely regulated by homeostasis, but something occurs in AD that disrupts homeostasis resulting in the accumulation of the protein within the brain and disrupting the neurons within its vicinity [11-12]. The increased amount of tau and the absence of β-amyloid in CSF have been proposed as possible diagnostic markers for AD, but results have not been consistent. The problem may be due to the presence of NFTs and SPs that increase in number during normal aging [13]. In order for the NFTs and SPs to be diagnostic of AD, they must be localized together in specific areas of the brain (neocortex and limbic region) [12]. SPs without NFTs are present in the same area in individuals with mild cognitive impairment (MCI) and in 27% of non-demented individuals greater then 75 years old [13].
A diagnosis of DLB is based on the presence of protein deposits called alpha-synuclein, which is referred to as Lewy Bodies, within brainstem and cortical neurons [6]. The cognitive deficit corresponds to the amount of Lewy Bodies within the brain.
FTLD is not characterized by a specific neuropathological feature. Typically, areas of the frontal/temporal cortices have neuronal loss, spongiform changes (microvacuolation) and severe astrocytic gliosis. The clinical symptoms in FTLD are dependent upon where the pathology is found rather than the type of pathology [7].
Currently, various neuropsychological tests are used to help diagnose dementia. For example, the Alzheimer's Disease Assessment Scale (ADAS)-cognitive subset is used to test the language ability (speech and comprehension), memory, ability to copy geometric figures and orientation to current time and place. The Folstein's Mini-Mental State Exam (MMSE), which also measures cognitive impairment, is an extensively validated test of orientation, short and long-term memory, praxis, language and comprehension. While these tests may indicate the level of cognitive impairment in an individual, they give no indication of whether the dementia may be caused by AD or by non-AD dementias.
It is commonly accepted that by the time any symptom is evident in any of the dementias described, irreversible neuronal loss has occurred [14]. MCI is characterized by a prominent impairment in memory with normal cognitive functions [15]. MCI is considered a transitional stage between normal aging and several types of dementia since a large proportion of individuals with MCI are later diagnosed with AD, DLB, or FTLD and all individuals with fully developed dementia first exhibit mild dementia symptoms similar to MCI [16].
There is a need to objectively differentiate the types of dementia from one another. Preferably, such a method would be specific, accurate, and efficient. Clearly, there is a pressing need for differential diagnosis of dementia prior to autopsy.
A biomarker that could detect neuropathological changes prior to clinical symptoms would be of enormous value. A consensus was reached in 1999 [17] as to what would be expected in a biomarker in AD:
1. Detect a fundamental feature of neuropathology
2. Diagnostic sensitivity of >80% for detecting AD
3. Specificity of >80% for distinguishing other dementias
4. Reliable
5. Reproducible
6. Noninvasive
7. Simple to perform
8. Inexpensive
The identification of AD-specific biomarkers in human serum would be extremely useful since it would be noninvasive and could be used to detect the presence of AD pathology prior to the manifestation of clinical symptoms and differentiate those patients who may have a different form of dementia but similar clinical symptoms.